Our objective is to define in genetic and molecular terms the mechanisms which regulate the expression of genetic information during cell growth and development. For this purpose, we have investigated two model systems which serve as analogs for erythrodifferentiation and immunodifferentiation. This work has led to development of techniques for gene isolation, has defined the sequence of the expression of the alpha and beta globin genes, has strengthened the somatic mutation hypothesis as a mechanism for generating immunoglobulin diversity and has led to the construction of safer vectors for use in the cloning of recombinant DNA molecules.